1. Field of the Invention
This invention relates to glycolipid methyl esters and in particular to a process for producing a glycolipid methyl ester having surface activity and wax-like properties and represented by formula (I), ##STR3## wherein R.sub.3 represents hydrogen or methyl, R.sub.4 represents a saturated or unsaturated hydrocarbon group having 12 to 16 carbon atoms when R.sub.3 is a hydrogen atom, and R.sub.4 represents a saturated or unsaturated hydrocarbon group having 11 to 15 carbon atoms when R.sub.3 is a methyl group.
2. Description of the Prior Art
Higher fatty acids esters of sucrose (popular name: sugar esters) and higher fatty acid esters of anhydrosorbitol (popular name: Span) have been widely used in the field of surface active agents, particularly as emulsifying agents. Such known esters are formed via the ester bonding between the hydroxy group of the sugar moiety and the higher fatty acid, and the surface activities of these esters are regulated depending on their ester values. Selective esterification of sugar at its desired position or positions is nearly impossible because many hydroxy groups exist in the sugar structure. In this situation, therefore, only mixtures of various ester isomers are utilized on an industrial basis. The hydrophilic properties of these esters significantly decrease as the ester values increase because the esters are formed by esterification of the hydrophilic and hydroxy groups of the sugar. Consequently, much difficulty is encountered with the emulsifying processes. Another problem is that the esters are chemically unstable because the ester bond of the hydroxy groups in the sugar and higher fatty acid is more easily hydrolyzed than that of the common fatty alcohol esters.
It has been reported by J. F. T. Spencer et al [Canadian Journal of Chemistry, 39, 846 (1961)] that a great quantity of Sophorolipid is produced by culturing Torulopsis bombicola.
Sophorolipid is a mixture of the compounds represented by the formulas (IIa) and (IIb),
______________________________________ ##STR4## (IIa) ##STR5## (IIb) IIa-1: R.sub.1 = R.sub.2 = COCH.sub.3 IIa-2: R.sub.1 = COCH.sub.3, R.sub.2 = H IIa-3: R.sub.1 = H, R.sub.2 = COCH.sub.3 IIa-4: R.sub.1 = R.sub.2 = H IIb-1: R.sub.1 = R.sub.2 = COCH.sub.3 IIb-2: R.sub.1 = COCH.sub.3, R.sub.2 = H IIb-3: R.sub.1 = H, R.sub.2 = COCH.sub.3 IIb-4: R.sub.1 = R.sub.2 = H ______________________________________
wherein R.sub.3 and R.sub.4 in formulas (IIa) and (IIb) are the same as defined above.
As can be seen from formulas (IIa) and (IIb), Sophoropilid is a mixture of many glycolipids, and its basic structure is that of a [(2'-O-.beta.-D-glycopyranosyl-.beta.-D-glycopyranosyl)oxy]-alkane acid or alkene acid which is obtained via the glycoside bonding between Sophorose and a long-chain fatty acid having a hydroxy group at the .omega. or .omega.-1 position.
The compound of the present invention possesses structural features which cannot be found in the conventional glycolipid-type surface active agents and which are characterized by the fact that a stable glycoside bond is formed by the hyroxyfatty acid and sugar and that the end group of the alkyl or alkenyl group is a reactive carboxyl group. The compound is higher in chemical stability than those having the conventional ester bonds because the hydrophobic alkyl or alkenyl group is linked to the hydrophilic group is sugar via the glycoside bond. Moreover, since the alkyl or alkenyl group having hydrophobic properties is occupied at its end by the reactive carboxyl group, it is possible to produce glycolipids possessing surface-active and wax-like properties which have wide application by modification of only the carboxyl group without the hydroxy group of the sugar moiety chemically modified.
However, the production of the compound of formula (I) from Sophorolipid obtained by fermentation involves some problems. Namely, Sophorolipid cannot be used as a starting material because it is a mixture of many homologs having a lactone ring, a free carboxyl group, acetyl groups and the like as shown in formulas (IIa) and (IIb). Accordingly, the compound of the formula (IIb-4) should be first produced by eliminating the acetyl groups and releasing the carboxyl group without destruction of the carbon framework. The compound of formula (IIb-4) is a highly viscous substance peculiar to a sugar compound, and therefore, can only be obtained with much difficulty by means of any conventional method.
When Sophorolipid is forcibly dispersed in water and an acid or alkali is added to the resulting suspension in an amount necessary for normal hydrolysis of the ester bond, a part of the deacetylated or deacylated compound, which is subject to partial hydrolysis, acts as an emulsifying agent and incorporates the unreacted substances into micelles which protects them against continued attack by the remaining acid or alkali, thereby resulting in incompleted hydrolytic reaction.
For instance, the reaction proceeds only to an extent of about 50%, even if a given amount of potassium hydroxide (0.25 part per one part of Sophorolipid) is added to an aqueous solution containing 20% of Sophorolipid and the resulting solution is hydrolyzed with heating for 6 hours. When hydrochloric acid is used in an amount of 5% instead of the alkali catalyst, hydrolysis is as incomplete as in the case where the alkali catalyst is employed. Moreover, partial cleavage of the glycosyl ether bond results and damage to the basic structure occurs.
If the reaction is completed under the above conditions, potassium hydroxide should be used in an amount of 0.25 part per one part of Sophorolipid, which amount is extremely great, and it is nearly impossible to separate the formed compound of the formula (IIb-4) from the reaction solution by any industrially acceptable process. That is, the compound of the formula (IIb-4) is readily soluble in water, but can be only dissolved in lower alcohols such as methanol and ethanol, or special expensive organic solvents such as pyridine, dimethylsulfoxide and dimethylformamide, which would create a serious obstacle to safety. The compound of the formula (IIb-4) has a viscosity of more than 100,000 cps at room temperature. Moreover, since the potassium acetate formed at the same time is also readily soluble in water and alcohols, it is necessary to forcibly eliminate water and extract the potassium acetate with any one of the above nitrogen-containing solvents, and subsequently distill off the solvent in order to separate the compound of formula (IIb-4). This process is unacceptable from an industrial standpoint. It is not impossible but very difficult to obtain the compound of the present invention by reacting the free carboxyl group of the compound having the formula (IIb-4) with methanol because the compound of the formula (IIb-4) has a high viscosity. Another reason is that since there is no proper solvent capable of dissolving the compound, the glycosyl ether bond is cleaved under strongly acidic conditions.
With the above-noted difficulties in mind, a study has been conducted in which it has been found that non-hydrous Sophorolipid having a low viscosity can be obtained by adding a small amount of a polyhydric alcohol represented by formulas (III) or (IV), ##STR6## wherein R.sub.5 represents a hydrogen atom or a methyl group, R.sub.6 and R.sub.7 represent a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and m and n represent integers from 1 to 6, to hydrated Sophorolipid which is a fermentation product of Torulopsis bombicola, and distilling off water under reduced pressure by application of heat. This discovery is the subject matter of co-pending application Ser. No. 928,964.
Further studies have been conducted for a method of producing a highly purified compound of the formula (I) from the reaction-completed mixture of Sophorolipid having a low viscosity and the polyhydric alcohol merely by distilling off solvent by subjecting the mixture to a methanolysis reaction by adding methanol and an acid to the mixture to deacetylate and cleave the lactone ring, and at the same time, methylate the free carboxy group. As a result, it has been found that the above reaction proceeds highly advantageously and that a compound of a high purity of the formula (I) can be obtained in a good yield merely by distilling off the solvent from the reaction complete mixture.